The dry dock at Garden Island in Sydney Harbour is one of Defence’s oldest pieces of infrastructure. So what happens there?

At the time of its construction, the naval dry dock in Sydney Harbour was the largest and most complex engineering task ever undertaken in Australia, surpassed only by the Snowy Mountains Hydro Scheme in the 1950s.

Officially known as the Captain Cook Graving Dock, the facility has been in operation since 1945 and is an impressive 345 metres by 45 metres, 14 metres deep and holds around 260 million litres of water.

Now more than 70 years old, the dry dock remains a key enabler to Navy ship repair and maintenance, with about seven Navy and six commercial vessels dry-docked at Garden Island in 2017 for periods from as little as two weeks to as long as three months.

The facility is managed by Thales Australia, which has operated and maintained the dry dock and its supporting systems since 1998, amassing a wealth of experience which has ensured continuity of operations and full availability of an essential component to Navy capability.

With the closure of Cairncross Dock in Brisbane, the dry dock at Garden Island is now the only one in Australia capable of holding the Navy’s larger Fleet units.

The need for a naval graving dock in Australia became evident in the 1930s in the years before the Second World War. The dock would provide the country with an important strategic asset - one with the ability to accommodate not only the largest warships but also merchant ships of great tonnage, and allow naval forces to be repaired and returned to sea in the shortest possible time.

Construction of the sandstone structure commenced in 1940 and at its peak had more than 4,000 people working day and night to complete the project.

Most of the machinery required for the dockyard was manufactured in Britain and shipped to Australia. This was a risky undertaking, given the possibility of attack from German U-boats, bombers and Japanese submarines during the long voyage, but fortunately only two shipments were lost through enemy action.

The first docking was the emergency docking of the aircraft carrier HMS Illustrious in March 1945, a vessel of similar size to the current day HMAS Canberra, which arrived 22 days before the official opening of the dock.

Newcomers to Garden Island are surprised to learn that the machinery used to flood and empty the dock is located underground and still in operation after 70 years.

Stepping into the inner workings of the dry dock is like walking into a mineshaft. The collection of antique machinery can only be reached after climbing down a steep staircase and navigating a series of narrow, damp passageways. Hard hats are essential as the space is cramped and many of the doorways and hatches are low by modern standards.

Rhys Cowling, the Thales Facilities and Infrastructure Manager said a prime example of the old dock equipment is the pair of flooding valves. At 6 feet in diameter, these valves were cast in Scotland by Glenfield and Kennedy Ltd from cast iron and delivered by sea.

“These valves now need to be replaced and currently there is no identified Australian capability to cast to this size,” he said.

“The design and fabrication of the valves is expected to take between six to nine months, followed by a complex installation process.

“To illustrate the complexity of the dock, the entire system has 45 valves of different sizes, all requiring careful management and ultimately, replacement,” he said.

The dry dock is emptied by three massive bronze pumps. These are Gwynne Ltd Invincible horizontal centrifugal pumps each driven by a 1200 horsepower/5kv electric motor and were also manufactured in the UK and shipped to Australia during the dark days of the Second World War.

Two pumps are used at any one time and possess the capacity to empty the dock in eight hours at a rate of 320,000 litres per minute, or the equivalent of an Olympic Pool in 37 seconds. These drainage pumps are being progressively refurbished.

For the uninitiated, the dry dock resembles a giant swimming pool which is separated from the ocean by an underwater gate, called a caisson.

“The caisson is in fact a submarine, which when the ballast tanks are filled with water sinks into position making up the fourth wall of the dock, separating the dock from the harbour,” Mr Cowling said.

“There are two caissons in operation which enable sections of the dock to be separately drained.”

The caissons are the oldest structural vessels in the Navy. They were designed by Vickers Armstrong of Barrow-in-Furness, (the famous British arms manufacturing firm) and were built on location by the Sydney Steel Company.

Made of welded steel and fitted with buoyancy tanks, tidal chambers and ballast tanks, they are designed to enable their floatation and sinking within the dock grooves as required.

During their construction they were also armoured with 50mm of hardened steel to provide protection against torpedoes, a real threat given the Midget Submarine attack in Sydney Harbour in June 1942 which sunk the ferry HMAS Kuttabul which was moored at Garden Island.

Maintaining a Second World War era asset is not without its challenges. The ageing dock, unique in Australia and critical to Navy capability, requires systematic refurbishment and upgrade to ensure it remains available for Navy into the next decades.

“Most of the equipment within the dock area is 75 years old, which presents maintenance and heritage challenges,” ,” Mr Cowling said.

“A large number of these items, many of which were built overseas, are reaching the end of their working life and require either a complete overhaul or replacement.”